[FEATURE] Session-only flag for typed /model <name> and /effort <name>

Problem Statement

Typing /model <name> or /effort <name> persists the choice to settings.json globally, with no session-only form. Only the interactive picker's s key avoids this — and even s only covers the model column; effort still persists (#86873).

Proposed Solution

Add a session-only flag to the typed form, e.g. /model <name> -s|--session / /effort <name> -s|--session, matching the picker's s semantics without requiring the picker. Precedent already exists in-TUI: /plugin tag takes [--push] [--dry-run] [-f|--force], so a -s|--session flag on /model//effort follows the same shape as -f|--force there.

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Additional Context

Related: #63413 (same ask, stalled since May), #78654 (persist-by-default polarity), #85774 (confirm-before-save alternative), #86873 (picker's s doesn't cover effort).